March, 1906.] Life Cycle of a Homosporous Pteridophyte. 
487 
pendent when the gametophyte dies. When normal conditions 
are present the development of the embryo always results in the 
death of the parent gametophyte. 
During the juvenile stage the young sporophyte usually has 
a type of leaves different from those of the mature condition, but 
gradually it becomer more and more like the mature fern and 
finally takes on the form normal to the species, in which condi- 
tion it may live to an indefinite age. The horizontal rhizome 
continues to develop and branch at the tip and if decay takes 
place at the back end vegetative propagation is accomplished 
and the result is a larger and larger number of independent 
individuals. 
The four main stages of a fern are therefore as follows: 
1. The sporophyte or fern plant proper. 
2. The nonsexual spores produced as the result of a reduc- 
tion division. 
3. The gametophyte or thallus plant. 
4. The oospore produced as the result of the conjugation of 
the egg and sperm. 
An interesting deviation from the usual type of alternation of 
generations is present in some ferns. In a few species a thallus 
may develop by vegetative propagation from the tissues of the 
sporophyte. This is called apospory. Or the sporophyte may 
develop directly from the tissues of the gametophyte and not 
from an egg. This is known as apogamy. The details of 
apogamous and ajjosporous structures have not been investi- 
gated to any great extent and there is still doubt as to whether 
or not a conjugation or a reduction takes jjlace. 
But if these important processes are omitted the change from 
gametO])hytic to sporophytic characters or vice versa may be 
explained in the following manner. It is evident that every cell 
in the gametophyte, or at the least the re])roductive ones, must 
also contain the hereditary characters which are present in the 
sporophyte; but these characters are for the time being dormant. 
In the same way every cell in the sporophyte must also possess 
the hereditary characters peculiar to the gametophyte. Now, 
in ordinary cases gametophytic characters become active and 
assert themselves only as the result of a reduction division and 
sporophytic hereditary tendencies are only apparent after the 
conjugation of the egg and sperm. But there may be other 
stimuli able to induce the change, a response to which in the 
case of apogamy causes the gametophytic hereditary tendencies 
to become dormant and the sporophytic tendencies to become 
active, thus producing a sporophytic shoot; or in aposporv, 
causes the sporophytic tendencies to become dormant while the 
gametophytic tendencies resume activity. 
